Gas sensors have applications spanning much of the human experience. Research, industrial, safety, consumer, medical, and military applications abound. Many of gases of interest have absorption spectral features in the mid infrared that can be used to detect and differentiate between these gases. The combination of atmospheric transparency, strong spectral signatures, and availability of efficient light sources and detectors makes the mid-infrared (mid-IR) an attractive wavelength region for gas spectroscopy. For the purpose of the present disclosure, the discussion below is referring to gases, while it would be easily discovered by a skilled artisan that all principles disclosed below can be equally applied to the detection of liquids and gels; the materials that can be all together characterized as non-solid substances.
A great many gas absorption spectroscopy systems have been developed. [“Differential absorption spectroscopy”, U. Platt, J. Stutz, Springer Verlag Berlin Heidelberg, 2008] Systems that measure the complete mid-IR absorption spectrum provide the most information, but they can be expensive, power hungry, and physically large. Although these systems can detect multiple gases, they are poorly suited for many gas detection applications, such as leaks detection, environment monitoring etc. where cost, power consumption, and size are paramount factors.
Alternatively, systems can be designed to use only two wavelengths: an absorption wavelength that is absorbed by the gas under investigation, and a reference wavelength that is not. A comparison of the intensity of the absorption and reference wavelengths reveals the presence and concentration of the target gas. For many applications, an additional requirement is that neither wavelength should be absorbed by standard atmospheric gases. This permits the detection of target gas(es) diluted in the atmosphere.
There are many ways to generate the absorption and reference wavelengths. One is to use a single broadband light source, such as a glow bar, and wavelength selective filters. The selective filters of each of the above wavelengths are altered using mechanical actuator system (for example, a rotating disc with holes where the filters are physically mounted). This approach is attractive in that the probe and reference wavelengths originate from the same physical space, and any variations in the input optical power can be expected to affect both absorption and reference wavelengths in a similar way. It is unattractive in that most of the broadband optical power is wasted and that only low frequency variations in the source intensity can be compensated for due to the slow mechanically actuated filter system.
Another option improves power efficiency by utilizing two independent and relatively narrow spectrum light sources. If the narrow light sources are Light Emitting Diodes (LEDs), then they can be alternated rapidly using a pseudorandom sequence. A correlation analysis of the detector signal with the pseudorandom sequence greatly reduces noise in this approach. Power efficiency is also improved due to the efficiency of LEDs compared to broadband sources and the fact that only useful wavelengths are generated.
The Gallium Antimonide (GaSb) material system can produce LEDs covering the spectral range of the infrared (IR) light with the wavelength from 2 to 12 urn, permitting the detection of many gases, including methane, a commercially and environmentally important gas.
The approach of using two narrow band light sources, such as LEDs, has some downsides as well. In particular, the optical complexity of the system increases, the light of the two wavelengths does not share the same optical path, and the sources operate at independent, possibly different temperatures. The increased optical complexity adversely impacts the cost and the size of the detector, and as a result of the entire system. Any variations in the optical alignment due to mechanical vibrations or thermal imbalance may produce measurement and detection errors. While these errors can be theoretically “averaged away” using correlation analysis, the result could be slower measurements and increased power consumption.
The independence and possible differing of the temperatures of the two light sources is a more fundamental problem. The efficiency and central emission wavelength of all existing LEDs is dependent on temperature; these effects are particularly evident for mid-IR LEDs since they use narrow band gap semiconductor materials whose properties are very sensitive to temperature. Further, the temperature of the LED is affected by self-heating from its own bias current. A pseudorandom variation in LED current results in a correlated pseudorandom variation in the wavelength of the emitted light and its intensity. A correlation analysis is then not effective in countering this subtle problem.
In the light of the above, the best possibility for an LED-based gas detection system would be to use a single narrow-band light source along with a single wideband detector. This design has many advantages, a simplified optical arrangement being the most evident. However, in order to achieve such system, that single narrow-band light source must emit two different wavelengths of light from the same physical space. LEDs can do this by means of extreme temperature shifts, but achieving these temperature shifts is slow and introduces errors due to the varying temperature of the LED. Pseudorandom correlation becomes impractical at the low speeds achievable with temperature based wavelength tuning.
There is a need, therefore, for a light source that would produce a dual color, narrow band light emission from the same physical space, with independent or correlated control of the light intensity of each color. It is further desired that the said dual color light source is simple, low cost and easy in both mounting and control. Each wavelength, or a “color”, of a dual light color source should respectively correspond to an absorption and transmission spectra of a gas targeted for detection in a certain gas detection system.
The present invention teaches the design of such dual color LED that emits the light of one of the two pre-selected wavelengths in the IR region of the spectra, depending on the polarity of the applied external electric bias. These and other advantages of the present invention will be more readily understood from the following discussion taken in conjunction with the accompanying drawings.